Slide Six is a slide show hosting service that offers you the ability to add your voice narrations without having to create, upload, and sync a separate audio file. Slide Six does this by allowing you to record your narration directly through the Slide Six site. To use the feature just upload your slide show and then record your voice as you go through your slide deck. Slide Six also allows you to upload attachments to complement your presentations. YouTube and Vimeo videos are supported within Slide Six.
Slide Six supports all Power Point formats, Open Office, PDF, and MOV presentations. You can try Slide Six without creating an account through the trial uploader.
Applications for Education
If you have students creating slide shows that are going to end up in a wiki or on a blog consider having them add audio narration to their presentations before embedding them into a blog or wiki.
There is one last noteworthy aspect of Slide Six for teachers that has nothing to do with the technology of Slide Six, but everything to do with the owner and creator Todd Sharp. Slide Six is a boot-strapped start-up and Todd is very responsive to customer questions. Follow him on Twitter and tell him I sent you.